Hydrostatic transmissions use a hydraulic fluid to transmit power from a power source (for example, an internal combustion engine) to a power output (for example, a final drive or a plurality of wheels). Hydrostatic transmissions are typically used in agricultural tractors and other off-highway equipment, for example, forklifts, excavators, earth moving machines, and other vehicles.
The major benefits of hydrostatic transmissions are a large range of continuously variable speed, a precise control of traction effort and speed, and high maneuverability. Each of these benefits is directly related to vehicle productivity. Other advantages include high power capability in a compact size, a fast response related to low inertia, maintaining a controlled speed regardless of load, high traction force at a low engine speed, flexibility in packaging, dynamic braking, and simplicity in reversing vehicle direction. Compared to traditional solutions, such as a hydrodynamic transmission with a torque converter, hydrostatic transmissions can provide improved performance. As a non-limiting example, a wheel loader application may require high maneuverability and a wide torque and speed conversion range.
Hydrostatic transmissions are not without their drawbacks, however. Hydrostatic transmissions tend to have a lower overall efficiency, increased maintenance costs, and increased initial investment cost compared to conventional gear transmissions. As a result, design considerations for a given application in a hydrostatic transmission are very important. As a non-limiting example, a hydrostatic transmission design can focus on one or more particular operating modes, such as low speed driving to provide maximum tractive effort, variable speed operation, or maximum speed operation. Focusing a design on an operating mode, will increase an overall efficiency of the transmission and proper sizing of transmission components will result in a more cost-effective solution.
A hydrostatic driveline can be divided into many standard categories based on the characteristics of the hydraulic pump and the hydraulic motor. The hydrostatic driveline can include a fixed displacement pump or a variable displacement pump and a fixed displacement motor or a variable displacement motor. A common combination amongst hydrostatic drivelines is a driveline configured with a variable displacement pump and a fixed displacement motor. In this combination, an output speed is controlled by varying a displacement of the pump.
To increase versatility of a hydrostatic driveline, such as including a high output capacity and a wide velocity of operational ranges, many alternative concepts of hydrostatic drivelines have been developed to meet such demands. One of the simplest and most common solutions is to use the hydrostatic transmission with a mechanical gearbox connected in series. However, such a driveline is typically inefficient as the driveline does not provide a direct drive mode of operation.
The direct drive mode of operation enables the use of one or more additional power paths between the power source and the power output. The additional power paths provides a wider range of output speeds compared to a hydrostatic power path alone, through the use of a bypass path that connects the power source “directly” through one or more gear ratio paths to the power output, without incurring losses which are inherent in the hydrostatic power path. “Direct drive” power paths are already well-known in the art. One such implementation of the direct drive mode of operation is performed by locking up a torque converter. Publications which are exemplary of the direct drive mode of operation are U.S. Pat. No. 5,946,983, assigned to Clark Hurth Components S.P.A., and U.S. Patent Application No. 2011/0030505 assigned to J.C. Bamford Excavators Limited and JCB Transmissions.
While the concept the direct drive mode of operation is widely understood, the present invention provides a unique approach to optimize drive ratios used with the power paths and arrangements of components that is effective in both cost and performance. It would be advantageous to develop a hydrostatic driveline that includes a direct drive capability, which offers the benefits of increased efficiency at a high speed operating mode while maintaining the benefits of a hydrostatic drive at a low speed operating mode.